Just found Dorotheas's brother, Louis Sievert in the Ellis Island Records at www.ellisislandrecords.org
Louis visited Germany in 1910 at the age of 74. Visited a cousin Benjt(sp?) Blaffert. (So apparently a sister of Louis's and Dorothea's father or mother married a Blaffert.) Record confirms that Louis was in Buffalo from 1850 to 1855 so 1850 appears like a bona fide arrival date for Dorothea and her parents in New York.